Moreover, this group of fungi provides scientists with valuable experimental material for investigations of the mechanisms which, although allowing their growth at moderately high temperatures, limit it beyond 60 to 62☌ ( 243). However, considering that the vast majority of eukaryotes cannot survive prolonged exposure to temperatures above 40 to 45☌ ( 8), the ability of some 30 species, out of approximately 50,000 recorded fungal species, to breach the upper temperature limit of eukaryotes is a phenomenon that deserves elucidation. Perhaps because of their moderate degree of thermophily and because their habitats are not exotic, thermophilic fungi have not received much publicity and attention. Thermophily in fungi is not as extreme as in eubacteria or archaea, some species of which are able to grow near or above 100☌ in thermal springs, solfatara fields, or hydrothermal vents ( 36, 45). Such fungi comprise thermophilic and thermotolerant forms, which are arbitrarily distinguished on the basis of their minimum and maximum temperature of growth ( 63): the thermophilic fungi have a growth temperature minimum at or above 20☌ and a growth temperature maximum at or above 50☌, and the thermotolerant forms have a temperature range of growth from below 20 to ∼55☌. Although rigorous data are lacking, it appears that eukaryotic thermophily involves several mechanisms of stabilization of enzymes or optimization of their activity, with different mechanisms operating for different enzymes.Īmong the eukaryotic organisms, only a few species of fungi have the ability to thrive at temperatures between 45 and 55☌. By contrast, the thermal stability of the few intracellular enzymes that have been purified is comparable to or, in some cases, lower than that of enzymes from the mesophilic fungi. Genes of thermophilic fungi encoding lipase, protease, xylanase, and cellulase have been cloned and overexpressed in heterologous fungi, and pure crystalline proteins have been obtained for elucidation of the mechanisms of their intrinsic thermostability and catalysis.
Some extracellular enzymes from thermophilic fungi are being produced commercially, and a few others have commercial prospects. Their extracellular enzymes display temperature optima for activity that are close to or above the optimum temperature for the growth of organism and, in general, are more heat stable than those of the mesophilic fungi.
The properties of their enzymes show differences not only among species but also among strains of the same species. Thermophilic fungi have a powerful ability to degrade polysaccharide constituents of biomass. Some species have the ability to grow at ambient temperatures if cultures are initiated with germinated spores or mycelial inoculum or if a nutritionally rich medium is used. Studies of their growth kinetics, respiration, mixed-substrate utilization, nutrient uptake, and protein breakdown rate have provided some basic information not only on thermophilic fungi but also on filamentous fungi in general. Thermophilic fungi can be grown in minimal media with metabolic rates and growth yields comparable to those of mesophilic fungi. This review, for the first time, compiles information on the physiology and enzymes of thermophilic fungi. However, thermophilic fungi are potential sources of enzymes with scientific and commercial interests. Although widespread in terrestrial habitats, they have remained underexplored compared to thermophilic species of eubacteria and archaea. As the only representatives of eukaryotic organisms that can grow at temperatures above 45☌, the thermophilic fungi are valuable experimental systems for investigations of mechanisms that allow growth at moderately high temperature yet limit their growth beyond 60 to 62☌.
Thermophilic fungi are a small assemblage in mycota that have a minimum temperature of growth at or above 20☌ and a maximum temperature of growth extending up to 60 to 62☌.
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